What is a primary key and foreign key?

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Primary Key and Foreign Key are two fundamental concepts in relational databases that help uniquely identify records and maintain relationships between tables.

🔹 Primary Key
A primary key is a column (or combination of columns) in a table that uniquely identifies each row. It must contain unique and non-null values.

• Example: In a Students table, student_id can be the primary key, ensuring each student is distinct.

• A table can have only one primary key, though it can consist of multiple columns (composite key).

🔹 Foreign Key
A foreign key is a column in one table that refers to the primary key in another table, creating a relationship between the two tables.

• Example: In an Enrollments table, student_id is a foreign key that references student_id in the Students table.

• Foreign keys help maintain referential integrity, ensuring that the value in the foreign key column must exist in the referenced primary key column.

🔹 Key Differences:

Feature	Primary Key	Foreign Key
Purpose	Uniquely identifies row	Links two tables
Uniqueness	Must be unique	Can repeat
Null Values	Cannot be null	Can be null (in some cases)

✅ Summary:
A primary key uniquely identifies a record within its own table, while a foreign key connects that record to data in another table, ensuring consistency and relational structure.

Read More :

What are aggregate functions?

What is normalization in databases?

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